Wiring substrate

ABSTRACT

A wiring substrate includes an insulating layer including inorganic filler particles and resin, and a conductor layer including a metal film formed on a surface of the insulating layer and having a conductor pattern. The inorganic filler particles include first inorganic filler particles such that each of the first inorganic filler particles has a portion exposed on the surface of the insulating layer and is at least partially separated from the resin, the conductor layer is formed such that a part of the metal film is between the first inorganic filler particles and the resin from the surface of the insulating layer and that a distance between the surface of the insulating layer and the surface of the insulating layer at a deepest part of the part of the metal film is in the range of 0.1 μm to 0.5 μm.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based upon and claims the benefit of priorityto Japanese Patent Application No. 2021-198078, filed Dec. 6, 2021, theentire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wiring substrate.

Description of Background Art

Japanese Patent Application Laid-Open Publication No. 2017-199703describes a wiring substrate that includes an insulating layer in whichinorganic insulating filler particles are contained in a thermosettingresin. The entire contents of this publication are incorporated hereinby reference.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a wiring substrateincludes an insulating layer including inorganic filler particles andresin, and a conductor layer including a metal film formed on a surfaceof the insulating layer and having a conductor pattern. The inorganicfiller particles include first inorganic filler particles such that eachof the first inorganic filler particles has a portion exposed on thesurface of the insulating layer and is at least partially separated fromthe resin, the conductor layer is formed such that a part of the metalfilm is between the first inorganic filler particles and the resin fromthe surface of the insulating layer and that the distance between thesurface of the insulating layer and the surface of the insulating layerat the deepest part of the part of the metal film is in the range of 0.1μm to 0.5 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein: and

FIG. 1 is a cross-sectional view illustrating an example of a wiringsubstrate according to an embodiment of the present invention;

FIG. 2 is a cross-sectional view schematically illustrating an exampleof a cross section near an interface between an insulating layer and aconductor layer in the wiring substrate according to an embodiment ofthe present invention;

FIG. 3 is an enlarged view of a portion (III) in FIG. 2 .

FIG. 4 is a cross-sectional view schematically illustrating anotherexample of a cross section near an interface between an insulating layerand a conductor layer in a wiring substrate according to an embodimentof the present invention;

FIG. 5 is a plan view schematically illustrating a surface of aninsulating layer in a wiring substrate according to an embodiment of thepresent invention;

FIG. 6 is a photographed image of a cross section near an interfacebetween an insulating layer and a conductor layer in a working exampleaccording to an embodiment of the present invention;

FIG. 7A is a cross-sectional view illustrating an example of amanufacturing process of a wiring substrate according to an embodimentof the present invention;

FIG. 7B is a cross-sectional view illustrating an example of amanufacturing process of a wiring substrate according to an embodimentof the present invention;

FIG. 7C is a cross-sectional view illustrating an example of amanufacturing process of a wiring substrate according to an embodimentof the present invention;

FIG. 7D is a cross-sectional view illustrating an example of amanufacturing process of a wiring substrate according to an embodimentof the present invention;

FIG. 7E is a cross-sectional view illustrating an example of amanufacturing process of a wiring substrate according to an embodimentof the present invention;

FIG. 7F is a cross-sectional view illustrating an example of amanufacturing process of a wiring substrate according to an embodimentof the present invention;

FIG. 7G is a cross-sectional view illustrating an example of amanufacturing process of a wiring substrate according to an embodimentof the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments will now be described with reference to the accompanyingdrawings, wherein like reference numerals designate corresponding oridentical elements throughout the various drawings.

A wiring substrate according to an embodiment of the present inventionis described with reference to the drawings. FIG. 1 is a cross-sectionalview illustrating a wiring substrate 100, which is an example of thewiring substrate of the embodiment. FIG. 2 schematically illustrates anenlarged cross section near an interface between an insulating layer 4and a conductor layer (metal film (3 a)) in the wiring substrate 100(for example, a portion (II) in FIG. 1 ). FIG. 3 illustrates a furtherenlarged view of a portion (III) of FIG. 2 . The wiring substrate 100 ismerely an example of the wiring substrate of the present embodiment. Alaminated structure, the number of conductor layers, and the number ofinsulating layers of the wiring substrate of the embodiment are notlimited to a laminated structure of the wiring substrate 100 of FIG. 1 ,the number of conductor layers 3 and the number of insulating layers 4included in the wiring substrate 100.

As illustrated in FIG. 1 , the wiring substrate 100 includes a coresubstrate 1, and build-up layers 2 that are respectively laminated ontwo main surfaces (a first surface (1 f) and a second surface (is)) ofthe core substrate 1 opposing each other in a thickness direction of thecore substrate 1. The core substrate 1 includes an insulating layer 12,and conductor layers 11 that are respectively formed on both sides ofthe insulating layer 12. The insulating layer 12 includes through-holeconductors 13 that penetrate the insulating layer 12 and connect theconductor layer 11 on the first surface (1 f) side and the conductorlayer 11 on the second surface (is) side to each other.

The build-up layers 2 that are respectively formed on the first surface(1 f) and the second surface (is) of the core substrate 1 each includean insulating layer 4 and a conductor layer 3 formed on a surface (4 a)of the insulating layer 4. The insulating layer 4 includes viaconductors 30 that penetrate the insulating layer 4 and connect to eachother the conductor layer 11 and the conductor layer 3 that are adjacentto each other via the insulating layer 4. Each o the build-up layers 2in the example of FIG. 1 includes one insulating layer 4 and oneconductor layer 3. However, the wiring substrate 100 of the presentembodiment may include build-up layers that each include any number ofinsulating layers and any number of conductor layers.

In the description of the embodiment, a side farther from the insulatinglayer 12 in a thickness direction of the wiring substrate 100 is alsoreferred to as an “upper side” or simply “upper,” and a side closer tothe insulating layer 12 is also referred to as a “lower side” or simply“lower.” Further, for the conductor layers and the insulating layers, asurface facing the opposite side with respect to the insulating layer 12is also referred to as an “upper surface,” and a surface facing theinsulating layer 12 side is also referred to as a “lower surface.”

Each of the insulating layers 4 and the insulating layer 12 contains anyinsulating resin. Examples of the insulating resin include an epoxyresin, a bismaleimide triazine resin (BT resin), a phenol resin, and thelike. Each of the insulating layers may contain a core material(reinforcing material) formed of a glass fiber or an aramid fiber. Asillustrated in FIGS. 2 and 3 , each of the insulating layers may furthercontain inorganic filler particles 5 formed of fine particles of silica(SiO2), alumina, mullite, or the like. In the present embodiment, atleast each of the insulating layers 4 contains multiple inorganic fillerparticles 5.

In the example of FIG. 1 , each of the conductor layers 3 includes alower layer formed of a metal film (3 a) formed on the surface (4 a) ofthe insulating layer 4, and an upper layer formed of a plating film (3b) formed on the metal film (3 a). On the other hand, each of theconductor layers 11 includes a lower layer formed of a metal foil (11 c)formed on a surface of the insulating layer 12, a middle layer formed ofa metal film (11 a) formed on the metal foil (11 c), and an upper layerformed of a plating film (11 b) formed on the metal film (11 a). Themetal film (3 a) and the metal film (11 a) are each, for example, anelectroless plating film or a sputtering film. The plating film (3 b)and the plating film (11 b) are each, for example, an electrolyticplating film. Each of the conductor layers 3 and the conductor layers 11is formed using any metal such as copper or nickel.

The via conductors 30 are integrally formed with the conductor layers 3.Therefore, the via conductors 30 are formed of any metal such as copperor nickel similar to the conductor layers 3 and each have a two-layerstructure including a metal film (3 a) and a plating film (3 b) similarto the conductor layers 3. On the other hand, the through-holeconductors 13 are integrally formed with the conductor layers 11.Therefore, the through-hole conductors 13 are formed of any metal suchas copper or nickel similar to the conductor layers 11. The through-holeconductors 13 are formed of the metal film (11 a) and the plating film(11 b) that respectively form the middle layer and the upper layer ofeach of the conductor layers 11.

Each of the conductor layers 3 and the conductor layers 11 includespredetermined conductor patterns. In the example of FIG. 1 , theconductor layers 3 include conductor pads 31. Electrodes of an activecomponent (not illustrated in the drawings) such as a semiconductorintegrated circuit device or a transistor, or a passive component (notillustrated in the drawings) such as an electrical resistance areconnected to the conductor pads 31. The conductor pads 31 may beconnected to another wiring substrate such as a motherboard of anelectrical device in which the wiring substrate 100 is used.

The conductor layer 3 on the first surface (1 f) side of the coresubstrate 1 includes multiple wiring patterns 32 adjacent to each other.In the wiring substrate 100, as will be described later, it is thoughtthat insulation between the conductor patterns included in the conductorlayer 3 is improved. That is, it is thought that a short circuit failurebetween the wiring patterns 32 is unlikely to occur. Therefore, themultiple wiring patterns 32 are formed with narrow wiring widths andintervals. For example, the multiple wiring patterns 32 may be formedaccording to a wiring rule of (5 μm)/(5 μm), regarding a wiring ruledefined by a combination (L/S) of a minimum wiring width (L) and aminimum wiring interval (S). Therefore, the conductor layer 3 includesthe multiple wiring patterns 32 having a (minimum wiring width(L))/(minimum wiring interval (S)) of (5 μm)/(5 μm).

A solder resist 6 is formed on the insulating layer 4 and the conductorlayer 3 on each of the first surface (1 f) side and the second surface(is) side of the core substrate 1. Openings exposing the conductor pads31 are provided in the solder resists 6. The solder resists 6 are eachformed of, for example, a photosensitive epoxy resin or polyimide resin,or the like. A surface protection film (not illustrated in the drawings)may be provided on surfaces of the conductor pads 31 exposed in theopenings of the solder resists 6 to prevent corrosion or oxidation ofthe surfaces of the conductor pads 31. The surface protection film mayhave, for example, a single-layer structure or a multilayer structureformed of Au, Ni/Au, Ni/Pd/Au or the like.

As illustrated in FIG. 2 , the surface (4 a) of the insulating layer 4has fine unevenness (4 u). As will be described later, the unevenness (4u) is formed by subjecting the surface (4 a) to a roughening treatmentusing, for example, a chemical method. The unevenness (4 u) may beformed by peeling off some of the multiple inorganic filler particles 5exposed on the surface (4 a) of the insulating layer 4 by the rougheningtreatment.

The insulating layer 4 of the wiring substrate 100 contains multipleinorganic filler particles 5 and a resin part 41 surrounding themultiple inorganic filler particles 5. The resin part 41 contains as amain component an epoxy resin, a BT resin, a phenol resin, or the likeexemplified above as a resin forming the insulating layer 4. By addingthe inorganic filler particles 5 formed of silica, alumina, or the liketo the resin, mechanical strength and/or thermal conductivity of theinsulating layer 4 may be increased. Further, by adjusting an additiveamount of the inorganic filler particles 5, it may be possible to adjusta thermal expansion coefficient of the insulating layer 4.

In the wiring substrate 100 of the present embodiment, the multipleinorganic filler particles 5 include first inorganic filler particles51. The first inorganic filler particles 51 are some inorganic fillerparticles among the multiple inorganic filler particles 5, and areinorganic filler particles that each include a portion exposed on thesurface (4 a) of the insulating layer 4 and are each at least partiallyseparated from the resin part 41 of the insulating layer 4. That is, atleast a part of a surface of each of the first inorganic fillerparticles 51 is separated from the resin part 41 and is not in contactwith the resin part 41. The surface of each of the first inorganicfiller particles 51 has a portion that is not in contact with the resinpart 41. For example, by dissolving a portion of the resin part 41 incontact with an inorganic filler particle 5, the inorganic fillerparticle 5 in contact with the dissolved resin turns into a state calleda first inorganic filler particle 51. As illustrated in FIGS. 2 and 3 ,the multiple inorganic filler particles 5 include the multiple firstinorganic filler particles 51.

That an inorganic filler particle 5 is “separated from the resin part41” means that a portion of the surface of the inorganic filler particle5 opposing (facing) the resin part 41 is not in contact with the resinpart 41. That is, a state is intended in which a portion of the surfaceof the inorganic filler particle 5 opposing the resin part 41 is not incontact with the resin part 41 even though it opposes the resin part 41.Therefore, an inorganic filler particle 5 of which only a portion of asurface opposing the conductor layer 3 (metal film (3 a)) is not incontact with the resin part 41 does not belong to the first inorganicfiller particles 51.

In the example of FIGS. 2 and 3 , the multiple inorganic fillerparticles 5 also include inorganic filler particles that are not thefirst inorganic filler particles 51. Among the multiple inorganic fillerparticles 5, inorganic filler particles other than the first inorganicfiller particles 51 are also referred to as second inorganic fillerparticles 52. Each of the second inorganic filler particles 52 is aninorganic filler particle of which an entire surface portion opposingthe resin part 41 is in contact with the resin part 41. Each of thesecond inorganic filler particles 52 is substantially entirelysurrounded by the resin part 41 and substantially entirely embedded inthe insulating layer 4.

Although the surface of each of the first inorganic filler particles 51includes a portion separated from the resin part 41, as illustrated inFIG. 3 , a portion of the surface of each of the first inorganic fillerparticles 51 may be in contact with the resin part 41. The surfaces ofat least some of the multiple first inorganic filler particles 51 arepartially in contact with the resin part 41 and are connected to theresin part 41. That is, at least some of the first inorganic fillerparticles 51 are connected to a main body portion of the insulatinglayer 4 (a portion other than the first inorganic filler particles 51 inthe insulating layer 4). Since such first inorganic filler particles 51are contained in the insulating layer 4, it is thought that adhesionstrength between the metal film (3 a), that is, the conductor layer 3,and the insulating layer 4 is improved. The reason for this is describedbelow.

Since a portion of the surface of each of the first inorganic fillerparticles 51 is separated from the resin part 41, as illustrated in FIG.3 , there is a gap (G) between each of the first inorganic fillerparticles 51 and the resin part 41, which forms the main body portion ofthe insulating layer 4. A gap (G) is formed, for example, by dissolvinga portion of the resin part 41 in contact with an inorganic fillerparticle 5. The gaps (G) exist on the surface (4 a) of the insulatinglayer 4. Then, the metal film (3 a) forming the conductor layer 3 isformed on the surface (4 a). Since the gaps (G) exist on the surface (4a), as illustrated in FIGS. 2 and 3 , a part (3 aa) of the metal film (3a) enters (the gaps (G)) between the first inorganic filler particles 51and the resin part 41 from the surface (4 a) of the insulating layer 4.The part (3 aa) of the metal film (3 a) enters or penetrates to a backside (a portion facing the resin part 41) of each of the first inorganicfiller particles 51 from the surface (4 a). That is, the part of theconductor layer 3 (the part (3 aa) of the metal film (3 a)) is formednot only on surfaces of the resin part 41 and the inorganic fillerparticles 5 facing the metal film (3 a), but is also formed between thefirst inorganic filler particles 51 and the resin part 41.

On the other hand, as described above, some of the first inorganicfiller particles 51 are connected to the resin part 41 that forms themain body portion of the insulating layer 4. Therefore, when the metalfilm (3 a) is formed in the gaps (G), it is thought that a substantialcontact area between the metal film (3 a) (a part of the conductor layer3) and the insulating layer 4 is increased. Further, it is thought thata movement of the metal film (3 a) formed in the gaps (G) from thesurface (4 a) of the insulating layer 4 in a peeling direction ishindered by the first inorganic filler particles 51. Therefore, it isthought that the adhesion strength between the metal film (3 a), thatis, the conductor layer 3, and the insulating layer 4 is improved.

From such a point of view of improving the adhesion strength between theconductor layer 3 and the insulating layer 4, the metal film (3 a) ispreferably formed using a method that allows the metal film (3 a) to bealso easily formed in the gaps (G). For example, the metal film (3 a)may be an electroless plating film formed by electroless plating. Inelectroless plating performed in a plating solution, it is thought thatthe metal film (3 a) is easily formed in the gaps (G) compared tosputtering or the like.

In this way, in the present embodiment, the metal film (3 a) is alsoformed between the first inorganic filler particles 51 and the resinpart 41. Therefore, as described above, it is thought that the adhesionstrength between the conductor layer 3 and the insulating layer 4 isimproved. However, when the metal film (3 a) has excessively enteredfrom the surface (4 a) of the insulating layer 4, in the patterning ofthe conductor layer 3 in which an unwanted portion of the metal film (3a) is removed, it may be difficult to remove the metal film (3 a) in aregion in which the conductor patterns such as the wiring patterns 32illustrated in FIG. 1 are not formed. For example, it may be possiblethat an etching solution that dissolves the metal film (3 a) cannot flowdeep into the gaps (G), and the metal film (3 a) remains in the gaps(G). In this way, when the metal film (3 a) remains, insulation maydeteriorate between the conductor patterns of the conductor layer 3, forexample, between adjacent wiring patterns 32, and, for example, a shortcircuit failure or the like may occur.

However, in the present embodiment, a distance (D) between a deepestpart (MD) of the part (3 aa) of the metal film (3 a) from the surface (4a) of the insulating layer 4 and the surface (4 a) is 0.1 μm or more and0.5 μm or less. That is, the insulating layer 4 and/or the metal film (3a) are formed such that a distance between the surface (4 a) and a pointfarthest from the surface (4 a) in the metal film (3 a) interposedbetween the resin part 41 of the insulating layer 4 and a firstinorganic filler particle 51 is 0.1 μm or more and 0.5 μm or less.

For example, as the resin forming the resin part 41 of the insulatinglayer 4, a resin is selected that contains, at an appropriate ratio, aneasily soluble component and a hardly soluble component with respect toa predetermined solvent. Then, by exposing the surface (4 a) of theinsulating layer 4 to the predetermined solvent, depths of the gaps (G)relative to the surface (4 a) is adjusted. Then, for example, a platingcondition for forming the metal film (3 a) is adjusted so as to fill thegaps (G) including the deepest parts (D) having the above-describeddistances (D) relative to the surface (4 a). When the distances (D) are0.1 μm or more, it is thought that the effect of improving the adhesionbetween the conductor layer 3 and the insulating layer 4 as describedabove is obtained. Further, when the distances (D) are 0.5 μm or less,it is thought that the metal film (3 a) is unlikely to remain in thegaps (G). Therefore, according to the present embodiment, it may bepossible to improve the adhesion strength between the insulating layerand the conductor layer while suppressing deterioration in insulationbetween the conductor patterns.

A “deepest part (MD)” is a point in the part (3 aa) of the metal film (3a) that has entered into a gap (G), the point being farthest from thesurface (4 a) of the insulating layer 4 in the thickness direction (theZ direction) of the wiring substrate 100. A “distance (D) between adeepest part (MD) and the surface (4 a)” is a length in the Z directionbetween the deepest part (MD) and the surface (4 a). The “surface (4 a)”in these definitions for a “deepest part (MD)” and a “distance (D)between a deepest part (MD) and the surface (4 a)” is a point on asurface of the resin part 41 that is in contact with the part (3 aa) ofthe metal film (3 a) as a target and is positioned farthest from thedeepest part (MD) in the Z direction.

When there are multiple first inorganic filler particles 51 as in theexample of FIGS. 2 and 3 , the part (3 aa) of the metal film (3 a)around at least one first inorganic filler particle 51 has a distance(D) of 0.1 nm or more and 0.5 nm or less between the deepest part (MD)and the surface (4 a). Distances (D) of all the deepest parts (MD) ofthe part (3 aa) of the metal film (3 a) surrounding the multipleinorganic filler particles 51 may be 0.1 μm or more and 0.5 μm or lessrelative to the surface (4 a).

The surface (4 a) of the insulating layer 4 includes multiple recesses(4 b) that are recessed toward the opposite side with respect to themetal film (3 a). Some of the first inorganic filler particles 51 havepartially entered into the recesses (4 b). A gap (G) forms a part of arecess (4 b). In the example of FIG. 3 , for the first inorganic fillerparticle 51 on the left side, the part (3 aa) of the metal film (3 a)enters into a deepest part of the recess (4 b). It may be possible thatthe adhesion strength between the conductor layer 3 and the insulatinglayer 4 is further improved.

There are also recesses (4 c), into which the first inorganic fillerparticles 51 have not entered, on the surface (4 a) of the insulatinglayer 4. Even in such recesses (4 c), the contact area between the metalfilm (3 a) and the insulating layer 4 is increased at least compared tothat in a case of a flat part. Therefore, it is thought that therecesses (4 c) that do not contain the first inorganic filler particles51 also contribute to the improvement of the adhesion strength betweenthe conductor layer 3 and the insulating layer 4. Further, since therecesses (4 c) are not blocked by the inorganic filler particles 5, inthe patterning of the conductor layer 3 in which an unwanted portion ofthe metal film (3 a) is removed, the metal film (3 a) in the recesses (4c) is easily exposed to, for example, an etching solution that dissolvesthe metal film (3 a). Therefore, it is thought that the metal film (3 a)is unlikely to remain in the recesses (4 c) in a region in which theconductor patterns of the conductor layer 3 are not formed. The recesses(4 c) that do not contain the first inorganic filler particles 51 may beformed, for example, by removing the inorganic filler particles 5exposed on the surface (4 a) of the insulating layer 4.

FIG. 4 schematically illustrates another example of a cross section nearan interface between the insulating layer 4 and the conductor layer 3 inthe wiring substrate of the present embodiment. FIG. 4 illustrates aregion near the interface between the insulating layer 4 and theconductor layer 3 corresponding to the region illustrated in FIG. 3 . Inthe example of FIG. 4 , most of a first inorganic filler particle (51 a)on the left side is embedded in the insulating layer 4. However, a gap(G) between the first inorganic filler particle (51 a) and the resinpart 41 is not formed up to a front end part (T) of the first inorganicfiller particle (51 a) on the opposite side with respect to theconductor layer 3. Therefore, the part (3 aa) of the metal film (3 a)also does not reach the front end part (T) of the first inorganic fillerparticle (51 a). As a result, the part (3 aa) of the metal film (3 a)illustrated in FIG. 4 also has a distance (D) of 0.1 μm or more and 0.5μm or less between the deepest part (MD) and the surface (4 a) of theinsulating layer 4. In this way, in the present embodiment, the part (3aa) of the metal film (3 a) entering the gaps (G) does not have to enterto the front end parts of the first inorganic filler particles on theopposite side with respect to the conductor layer.

FIG. 5 schematically illustrates the surface (4 a) of the insulatinglayer 4 of the wiring substrate 100. Although FIG. 5 is a plan view, thefirst inorganic filler particles 51 are hatched with diagonal lines forease of understanding. As illustrated in FIG. 5 , the multiple firstinorganic filler particles 51 are exposed on the surface (4 a) of theinsulating layer 4. Further, the multiple recesses (4 b) and themultiple recesses (4 c) are formed on the surface (4 a). Some of themultiple first inorganic filler particles 51 have entered into therecesses (4 b). Recesses (4 b) illustrated with broken lines arerecesses (4 b) hidden by the first inorganic filler particles 51.

In the wiring substrate of the present embodiment, the surface (4 a) ofthe insulating layer 4 may include a region in which there are 5 or morefirst inorganic filler particles 51 per 10 μm. For example, in FIG. 5 ,a region (R) of which a contour is indicated with a one-dot chain lineincludes five first inorganic filler particles 51 aligned along alongitudinal direction of the region (R). Here, the region (R) has alength shorter than 10 μm in the longitudinal direction. That is, thereare at least 5 first inorganic filler particles 51 per 10 μm in theregion (R). The surface (4 a) of the insulating layer 4 in the exampleof FIG. 5 includes a region, such as the region (R), in which there are5 or more first inorganic filler particles 51 per 10 μm (a formingdensity of five first inorganic filler particles 51 per 10 μm is alsoreferred to as a “density (C)” in the following). In the example of FIG.5 , since the first inorganic filler particles 51 exist on the surface(4 a) of the insulating layer 4 at a density equal to or higher than thedensity (C), a sufficient effect of improving the adhesion strengthbetween the conductor layer 3 and the insulating layer 4 described abovedue to the first inorganic filler particles 51 is obtained.

A region in which the first inorganic filler particles 51 exist at adensity equal to or higher than the density (C) preferably exists on thesurface (4 a) at an interface between the insulating layer 4 and aconductor pattern of the conductor layer 3 (see FIG. 1 ). However, whena region that includes the first inorganic filler particles 51 at adensity equal to or higher than the density (C) exists somewhere on thesurface (4 a), it is thought that a region that includes the firstinorganic filler particles 51 at a density close to the density (C) alsoexists at an interface portion with the conductor layer 3 on the surface(4 a). Therefore, the surface (4 a) has a region that includes the firstinorganic filler particles 51 at a density equal to or higher than thedensity (C) at any location.

As described above, in the present embodiment, since the insulatinglayer 4 contains the multiple inorganic filler particles 5 including thefirst inorganic filler particles 51 and a part of the metal film (3 a)is formed between the first inorganic filler particles 51 and the resinpart 41, it is thought that the adhesion strength between the conductorlayer 3 (see FIG. 1 ) and the insulating layer 4 is improved. Inaddition, when the surface (4 a) of the insulating layer 4 includes thefirst inorganic filler particles 51 at a density equal to or higher thanthe density (C), it is thought that the adhesion strength between theconductor layer 3 and the insulating layer 4 is further improved. Evenwhen the wiring widths of the wiring patterns 32 (see FIG. 1 ) becomesmaller along with miniaturization of the conductor patterns, it isthought that the wiring patterns 32 are unlikely to peel off from theinsulating layer 4. Further, even when the additive amount of theinorganic filler particles 5 is increased in order to reduce the thermalexpansion coefficient of the insulating layer 4, it is thought that thewiring patterns 32 are unlikely to peel off from the insulating layer 4.

On the other hand, in the wiring substrate of the present embodiment,the surface (4 a) of the insulating layer 4 may have an arithmetic meanroughness (Ra) of 0.05 μm or more and 0.5 μm or less. When the surface(4 a) of the insulating layer 4 has such a relatively small surfaceroughness, an unwanted portion (a portion that does not form a conductorpattern of the conductor layer 3) of the metal film (3 a) (see FIG. 1 )formed on the surface (4 a) is easily removed as intended. That is,since a deep recess that makes removal of the metal film (3 a) formedtherein difficult is unlikely to exist on the surface (4 a), an unwantedportion of the metal film (3 a) is appropriately removed, for example,by etching. Therefore, the insulation between the conductor patterns(for example, the wiring patterns 32 and the like) of the conductorlayer 3 is unlikely to deteriorate, and a short circuit failure isunlikely to occur.

Further, since the surface roughness of the surface (4 a) of theinsulating layer 4 is relatively small, it is thought that a surfaceroughness of the conductor layer 3 on the insulating layer 4 side isalso relatively small. Therefore, for example, in transmission of a highfrequency signal, even when a transmission signal is affected by a skineffect, it is thought that deterioration in transmission characteristicsor the like due to a substantial increase in impedance is unlikely tooccur.

In the present embodiment, regardless of the density of the firstinorganic filler particles 51 and the surface roughness of theinsulating layer 4, distances of the deepest parts (MD) (see FIG. 3 ) ofthe part (3 aa) of the metal film (3 a) around the first inorganicfiller particles 51 relative to the surface (4 a) of the insulatinglayer 4 are 0.1 μm or more and 0.5 μm or less. Therefore, it is thoughtthat a short circuit failure between the conductor patterns of theconductor layer 3 is suppressed.

The surface (4 a) of the insulating layer 4 has an arithmetic meanroughness (Ra) of 0.05 μm or more and 0.5 μm or less as an average valueof the entire surface (4 a). Therefore, the surface (4 a) has, forexample, an arithmetic mean roughness (Ra) of 0.05 μm or more and 0.5 μmor less for a surface roughness measured from one edge of the surface (4a) to the other edge on the opposite side with respect to the one edge.That is, the surface (4 a) of the insulating layer 4 has an arithmeticmean roughness (Ra) of 0.05 μm or more and 0.5 μm or less as a surfaceroughness including also the first inorganic filler particles 51. Theaverage value of the arithmetic mean roughness (Ra) of the entiresurface (4 a) may be obtained as an average value of arithmetic meanroughnesses (Ra) measured at all five places including four cornerportions and a center portion of the surface (4 a).

For example, when the conductor patterns such as the wiring patterns 32are formed according to a wiring rule of (15 μm)/(15 μm), even when thesurface (4 a) of the insulating layer 4 has an arithmetic mean roughness(Ra) of 0.5 μm, it is thought that a short circuit failure is unlikelyto occur between the conductor patterns. On the other hand, when thesurface (4 a) of the insulating layer 4 has an even smaller arithmeticmean roughness (Ra) of 0.05 μm, even when the conductor patterns such asthe wiring patterns 32 are formed according to the wiring rule of (3μm)/(3 μm) described above, it is thought that a short circuit failureis unlikely to occur between the conductor patterns. Therefore, theconductor patterns such as the wiring patterns 32 included in theconductor layer 3 has a minimum wiring width (L) and a minimum wiringinterval (S) each of 3 μm or more and 15 μm or less.

A content rate of the multiple inorganic filler particles in theinsulating layer 4 is, for example, 65% or more and 80% or less. Whenthe inorganic filler particles 5 exist in the insulating layer 4 at sucha content rate, it is thought that the first inorganic filler particles51 are likely to exist at a density equal to or higher than the density(C) described above, and that a predetermined function due to the resinpart 41 such as a binding function between the inorganic fillerparticles 5 is appropriately exhibited. In FIGS. 2-5 , each of theinorganic filler particles 5 is drawn as having a spherical shape.However, each of the inorganic filler particles 5 may have any shape.The inorganic filler particles 5 have particle sizes of, for example,about 0.01 μm-5 μm.

FIG. 6 shows a photographed image of a cross section near an interfacebetween the insulating layer 4 and the metal film (3 a) (which forms theconductor layer 3) in a working example of the wiring substrate of theembodiment.

As shown in FIG. 6 , the insulating layer 4 contains the multipleinorganic filler particles 5, and some (the first inorganic fillerparticles 51) of the multiple inorganic filler particles 5 each have aportion that is separated from the resin part 41 of the insulating layer4. The insulating layer 4 contains the multiple first inorganic fillerparticles 51. The surface (4 a) of the insulating layer 4 includes therecesses (4 b) into which the first inorganic filler particles 51 haveentered and the recesses (4 c) that do not contain the first inorganicfiller particles 51.

The metal film (3 a) is also formed between the first inorganic fillerparticles 51 and the resin part 41. The first inorganic filler particles51 appear to be completely separated from the resin part 41. However,the first inorganic filler particles 51 may be connected to the resinpart 41 in a region not photographed. As shown in the working example ofFIG. 6 , in the present embodiment, a part of the metal film (3 a)enters between the first inorganic filler particles 51 and the resinpart 41 of the insulating layer 4. Further, in the present embodiment,the distances (D) of the deepest parts of the metal film (3 a)interposed between the first inorganic filler particles 51 and the resinpart 41 relative to the surface (4 a) of the insulating layer 4 are 0.1μm or more and 0.5 μm or less. Therefore, good adhesion strength betweenthe conductor layer 3 (metal film (3 a)) and the insulating layer 4 andgood insulation between the conductor patterns included in the conductorlayer 3 are obtained.

Next, an example of a method for manufacturing the wiring substrate ofthe embodiment is described with reference to FIGS. 7A-7G, using thewiring substrate 100 of FIG. 1 as an example.

As illustrated in FIG. 7A, a starting substrate (for example, adouble-sided copper-clad laminated plate) that includes an insulatinglayer, which is to become the insulating layer 12 of the core substrate1, and the metal foils (11 c) that are respectively laminated on bothsides of the insulating layer is prepared, and the conductor layers 11and the through-hole conductors 13 of the core substrate 1 are formed.Through holes are formed at formation positions of the through-holeconductors 13, for example, by drilling, and the metal film (11 a) isformed in the through holes and on the metal foils (11 c), for example,by electroless plating. Then, the plating film (11 b) is formed byelectrolytic plating using the metal film (11 a) as a power feedinglayer. As a result, the conductor layers 11, each having a three-layerstructure, and the through-hole conductors 13 are formed. After that,the core substrate 1 having predetermined conductor patterns is obtainedby patterning the conductor layers 11 using a subtractive method.

As illustrated in FIGS. 7B and 7C, the insulating layer 4 containing themultiple inorganic filler particles 5 and the resin part 41 surroundingthe multiple inorganic filler particles 5 is formed. FIG. 7C illustratesan enlarged cross section of the insulating layer 4 near the surface (4a) in the state of FIG. 7B.

In the formation of the insulating layers 4, for example, a film-likeepoxy resin is laminated on both sides of the core substrate 1 and heatand pressure are applied thereto. As the resin forming the insulatinglayer 4, for example, a resin is selected that contains, at anappropriate ratio, an easily soluble component and a hardly solublecomponent with respect to a solvent used in a desmear treatment or aroughening treatment performed in a subsequent process. By doing so, thegaps (G) (see FIG. 7D) (to be described later) having moderate depthsare formed.

In the formation of the insulating layers 4, for example, a film-likeresin in which the multiple inorganic filler particles 5 are containedin a resin such as an epoxy resin is used. For example, an epoxy resincontaining the inorganic filler particles 5 at a content rate of 65%-80%is used. The resin part 41 of the insulating layer 4 is formed by aresin component formed of an epoxy resin or the like in the film-likeresin. In the insulating layer 4, through holes (30 a) for forming thevia conductors are formed by, for example, irradiation with CO₂ laser.

As illustrated in FIG. 7C, the multiple inorganic filler particles 5exist in the insulating layer 4 and near the surface (4 a) of theinsulating layer 4. Some of the multiple inorganic filler particles 5are substantially entirely or partially exposed from the surface (4 a)of the insulating layer 4. As illustrated in FIG. 7C, until after theformation of the through holes (30 a), the insulating layer 4 has asubstantially flat surface (4 a) except for portions where the inorganicfiller particles 5 exist.

After the formation of the through holes (30 a), when necessary, adesmear treatment is performed in which resin residues (smears)generated by the formation of the through holes (30 a) are removed. Forexample, the smears in the through holes (30 a) are removed by exposinginner walls of the through holes (30 a) to a processing liquid such asan alkaline permanganate solution. In the desmear treatment, the surface(4 a) of the insulating layer 4 is also exposed to the processingliquid.

As illustrated in FIG. 7D, the surface (4 a) of the insulating layer 4is roughened to have a predetermined surface roughness. Similar to FIG.7C, FIG. 7D illustrates an enlarged cross section near the surface (4 a)of the insulating layer 4. By the roughening of the surface (4 a), theadhesion strength between the insulating layer 4 and the conductor layer3 (see FIG. 7G) formed on the surface (4 a) is improved to some extent.

For example, the surface (4 a) of the insulating layer 4 is subjected toan alkaline permanganate solution treatment using a processing liquidsimilar to that used in the above-described desmear treatment. Thesurface (4 a) of the insulating layer 4 may be roughened to a desiredsurface roughness in the desmear treatment described above.

As illustrated in FIG. 7D, the first inorganic filler particles 51,which are inorganic filler particles partially separated from the resinpart 41 of the insulating layer 4, are provided by a rougheningtreatment and/or a desmear treatment. On the other hand, in theinsulating layer 4, the second inorganic filler particles 52, which areinorganic filler particles other than the first inorganic fillerparticles 51, remain in the state before the surface (4 a) is roughened.In the example of FIG. 7D, for example, due to the roughening of thesurface (4 a), for example, the inorganic filler particles 5 existing onthe surface (4 a) are detached and the recesses (4 c) are formed.

The first inorganic filler particles 51 are provided by forming apartial gap (G) between each of some inorganic filler particles 5, whichare among the multiple inorganic filler particles 5 and exist on thesurface (4 a) of the insulating layer 4, and the resin part 41. That is,in the roughening treatment and/or the desmear treatment of the surface(4 a) of the insulating layer 4, the resin part 41 forming the surface(4 a) of the insulating layer 4 is selectively dissolved, and thedissolved portion is removed. By selectively removing the resin part 41around the inorganic filler particles 5 existing near the surface (4 a),the gaps (G) are formed between the inorganic filler particles 5 and theresin part 41, and these inorganic filler particles 5 turn into thefirst inorganic filler particles 51 At the same time, the recesses (4 b)including the gaps (G) are formed.

In manufacturing the wiring substrate of the embodiment, gaps (G) havingdepths (D1) (distances of deepest parts (MD1) of the gaps (G) from thesurface (4 a) of the insulating layer 4) of 0.1 μm or more and 0.5 μm orless from the surface (4 a) of the insulating layer 4 are formed. Forexample, as the resin forming the resin part 41, as described above, byselecting a resin containing an easily soluble component and a hardlysoluble component at an appropriate ratio, the depths of the gaps (G)are adjusted.

In the roughening of the surface (4 a) of the insulating layer 4, thesurface (4 a) is roughened to have an arithmetic mean roughness (Ra) of,for example, 0.05 μm or more and 0.5 μm or less. The surface (4 a) maybe roughened to have an arithmetic mean roughness (Ra) of 0.05 μm ormore and 0.5 μm or less and include a region in which 5 or more firstinorganic filler particles 51 exist per 10 μm.

The roughening treatment of the surface (4 a) of the insulating layer 4may be performed using a common procedure and under a common condition.However, for example, in a roughening treatment based on an alkalinepermanganate solution treatment, a swelling treatment using a swellingliquid, an oxidation treatment using an oxidizing agent (substantialroughening treatment), and a neutralization treatment using aneutralizing liquid may be performed. As the swelling liquid, forexample, a sodium hydroxide solution or a potassium hydroxide solutionis used. The insulating layer 4 is exposed to the swelling liquid at apredetermined temperature for a predetermined time.

As the oxidizing agent, for example, an alkaline permanganate solutioncontaining permanganate at a predetermined concentration is used. Theinsulating layer 4 is exposed to the solution at a predeterminedtemperature for a predetermined time. As the neutralizing liquid, anacidic aqueous solution is used. The surface (4 a) of the insulatinglayer 4 roughened by the oxidation treatment is exposed to theneutralizing liquid at a predetermined temperature for a predeterminedtime. For example, conditions such as a treatment time, a temperature ofa treatment liquid, and/or a concentration of a main component in thetreatment liquid in each treatment during the series of rougheningtreatments are appropriately selected. As a result, the surface (4 a) ofthe insulating layer 4 is roughened such that the surface (4 a) has thedesired arithmetic mean roughness (Ra) described above, and preferably,in at least a portion of the surface (4 a), the first inorganic fillerparticles 51 are provided at a desired density equal to or higher thanthe density (C) (five first inorganic filler particles 51 per 10 μm).Further, by selecting these conditions, gaps (G) having depths (D1) of0.1 μm or more and 0.5 μm or less may be formed. After the roughening ofthe surface (4 a) of the insulating layer 4, cleaning may be performed.For example, the wiring substrate during manufacturing is cleaned byultrasonic cleaning. By ultrasonic cleaning, a state of the surface (4a) of the insulating layer 4 may be further brought closer to a desiredstate. For example, it is possible that excessive first inorganic fillerparticles 51 on the surface (4 a) are removed by ultrasonic cleaning,and instead, the recesses (4 c) are formed. Further, the resin part 41excessively existing on the surface (4 a) may be removed.

As illustrated in FIGS. 7E-7G, the metal film (3 a) is formed on thesurface (4 a) of the insulating layer 4, and the gaps (G) between thefirst inorganic filler particles 51 and the resin part 41 are filledwith a part of the metal film (3 a). Then, the conductor layer 3 isformed. In the example of FIGS. 7E-7G, the conductor layer 3 is formedusing a semi-additive method. FIG. 7F illustrates an enlarged crosssection of an interface portion between the insulating layer 4 and themetal film (3 a) in the state of FIG. 7E.

The metal film (3 a) is formed on the surface (4 a) of the insulatinglayer 4 and on the entire inner walls of the through holes (30 a) usingany metal such as copper or nickel. Further, as illustrated in FIG. 7F,the metal film (3 a) is also formed in the gaps (G) between the firstinorganic filler particles 51 and the resin part 41 of the insulatinglayer 4. By forming the metal film (3 a) also in the gaps (G) betweenthe first inorganic filler particles 51 and the resin part 41, theadhesion strength between the insulating layer 4 and the conductor layer3 (see FIG. 7G) is improved. Then, the part of the metal film (3 a)filling the gaps (G) with depths (D1) (see FIG. 7D) in theabove-described range has distances (D) of 0.1 μm or more and 0.5 μm orless between the deepest parts (MD) thereof and the surface (4 a) of theinsulating layer 4. Therefore, in removing an unwanted portion of themetal film (3 a) in a subsequent process, the unwanted portion of themetal film (3 a) to be removed is unlikely to remain.

The metal film (3 a) is formed, for example, using any method such aselectroless plating or sputtering. However, the metal film (3 a) ispreferably formed by electroless plating. In the formation of the metalfilm (3 a) by electroless plating, the metal film (3 a) is easily formedalso in the gaps (G) between the first inorganic filler particles 51 andthe resin part 41 of the insulating layer 4.

As illustrated in FIG. 7G, after the formation of the metal film (3 a),the plating film (3 b) in a desired pattern is formed on the metal film(3 a). The plating film (3 b) is formed by pattern plating includingelectrolytic plating using the metal film (3 a) as a power feedinglayer. As a result, the via conductors 30 are formed in the throughholes (30 a). The plating film (3 b) is formed, for example, using thesame material as that of the metal film (3 a) such as copper or nickel.

After that, an unwanted portion of the metal film (3 a) that is notcovered by the plating film (3 b) is removed, for example, by etching.As a result, the conductor layer 3 that has a two-layer structure andincludes the desired conductor patterns such as the conductor pads 31and the wiring patterns 32 is formed. In the present embodiment, asdescribed above, since the distances between the deepest parts (MD) ofthe part of the metal film (3 a) (see FIG. 7F) and the surface (4 a) ofthe insulating layer 4 are less than 0.5 μm, the unwanted portion of themetal film (3 a) is easily and properly removed.

After that, the solder resist 6 (see FIG. 1 ) is formed, for example, byforming a resin layer containing a photosensitive epoxy resin or thelike on the conductor layer 3 and the insulating layer 4. The openingsexposing the conductor pads 31 are provided in the solder resist 6., forexample, by exposure and development. The surface protection film (notillustrated in the drawings) may be formed on the conductor pads 31 byelectroless plating, solder leveling, or the like. Through the aboveprocesses, the wiring substrate 100 in the example of FIG. 1 iscompleted.

The wiring substrate of the embodiment is not limited to those havingthe structures illustrated in the drawings and those having thestructures, shapes, and materials exemplified herein. As describedabove, the wiring substrate of the embodiment may have any laminatedstructure. For example, the wiring substrate of the embodiment may be acoreless substrate that does not include a core substrate. The wiringsubstrate of the embodiment may have any number of conductor layers andany number of insulating layers. The conductor layer 3 does not have toinclude a plating film (3 b) formed of an electrolytic plating film, andmay include, for example, only the metal film (3 a) formed of anelectroless plating film.

Japanese Patent Application Laid-Open Publication No. 2017-199703describes a wiring substrate that includes an insulating layer in whichinorganic insulating filler particles are contained in a thermosettingresin. A surface of the insulating layer is roughened to an extent thatthe inorganic insulating filler particles are not exposed. Anelectroless copper plating film is formed on the roughened surface.After an electrolytic copper plating film of a predetermined pattern isformed on the electroless copper plating film, an exposed portion of theelectroless copper plating film is removed.

In the wiring substrate described in Japanese Patent ApplicationLaid-Open Publication No. 2017-199703, the electroless copper platingfilm is formed on the uneven surface of the insulating layer afterroughening, and the insulating layer and the electroless copper platingfilm are anchored by an anchor effect. However, when the surfaceroughness of the insulating layer is small, a sufficient anchor effectis not obtained, and adhesion strength between the insulating layer andthe electroless copper plating film may decrease. On the other hand,when the surface roughness of the insulating layer is large, theelectroless copper plating film formed deep in the irregularities on thesurface of the insulating layer may remain without being removed in aportion exposed from the electrolytic copper plating film.

A wiring substrate according to an embodiment of the present inventionincludes: an insulating layer that contains multiple inorganic fillerparticles and a resin part surrounding the multiple inorganic fillerparticles; and a conductor layer that includes a metal film formed on asurface of the insulating layer and includes a predetermined conductorpattern. The multiple inorganic filler particles include first inorganicfiller particles that each include a portion exposed on the surface andare each at least partially separated from the resin part. A part of themetal film enters between the first inorganic filler particles and theresin part from the surface. A distance between a deepest part of thepart of the metal film from the surface and the surface is 0.1 μm ormore and 0.5 μm or less.

According to an embodiment of the present invention, it may be possibleto improve adhesion strength between an insulating layer and a conductorlayer while suppressing deterioration in insulation between conductorpatterns.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A wiring substrate, comprising: an insulating layer comprisinginorganic filler particles and resin; and a conductor layer comprising ametal film formed on a surface of the insulating layer and having aconductor pattern, wherein the inorganic filler particles include firstinorganic filler particles such that each of the first inorganic fillerparticles has a portion exposed on the surface of the insulating layerand is at least partially separated from the resin, the conductor layeris formed such that a part of the metal film is between the firstinorganic filler particles and the resin from the surface of theinsulating layer and that a distance between the surface of theinsulating layer and the surface of the insulating layer at a deepestpart of the part of the metal film is in a range of 0.1 μm to 0.5 μm. 2.The wiring substrate according to claim 1, wherein the insulating layeris formed such that the surface of the insulating layer includes aregion in which 5 or more of the first inorganic filler particles existper 10 μm.
 3. The wiring substrate according to claim 1, wherein theconductor layer has a plurality of wiring patterns having a minimumwiring interval in a range of 3 μm to 15 μm.
 4. The wiring substrateaccording to claim 3, wherein the conductor layer is formed such that aminimum wiring width of the wiring patterns is in a range of 3 μm to 15μm.
 5. The wiring substrate according to claim 1, wherein the conductorlayer is formed such that the metal film is an electroless plating film.6. The wiring substrate according to claim 1, wherein the insulatinglayer is formed such that a content rate of the inorganic fillerparticles in the insulating layer is in a range of 65% to 80%.
 7. Thewiring substrate according to claim 1, wherein the insulating layer hasrecesses on the surface of the insulating layer and is formed such thatthe first inorganic filler particles is partially in the recesses on thesurface of the insulating layer and that the part of the metal film isin deepest parts of the recesses.
 8. The wiring substrate according toclaim 1, wherein the insulating layer is formed such that the surface ofthe insulating layer has an arithmetic mean roughness in a range of 0.05μm to 0.5 μm.
 9. The wiring substrate according to claim 2, wherein theconductor layer has a plurality of wiring patterns having a minimumwiring interval in a range of 3 μm to 15 μm.
 10. The wiring substrateaccording to claim 9, wherein the conductor layer is formed such that aminimum wiring width of the wiring patterns is in a range of 3 μm to 15μm.
 11. The wiring substrate according to claim 2, wherein the conductorlayer is formed such that the metal film is an electroless plating film.12. The wiring substrate according to claim 2, wherein the insulatinglayer is formed such that a content rate of the inorganic fillerparticles in the insulating layer is in a range of 65% to 80%.
 13. Thewiring substrate according to claim 2, wherein the insulating layer hasrecesses on the surface of the insulating layer and is formed such thatthe first inorganic filler particles is partially in the recesses on thesurface of the insulating layer and that the part of the metal film isin deepest parts of the recesses.
 14. The wiring substrate according toclaim 2, wherein the insulating layer is formed such that the surface ofthe insulating layer has an arithmetic mean roughness in a range of 0.05μm to 0.5 μm.
 15. The wiring substrate according to claim 3, wherein theconductor layer is formed such that the metal film is an electrolessplating film.
 16. The wiring substrate according to claim 3, wherein theinsulating layer is formed such that a content rate of the inorganicfiller particles in the insulating layer is in a range of 65% to 80%.17. The wiring substrate according to claim 3, wherein the insulatinglayer has recesses on the surface of the insulating layer and is formedsuch that the first inorganic filler particles is partially in therecesses on the surface of the insulating layer and that the part of themetal film is in deepest parts of the recesses.
 18. The wiring substrateaccording to claim 3, wherein the insulating layer is formed such thatthe surface of the insulating layer has an arithmetic mean roughness ina range of 0.05 μm to 0.5 μm.
 19. The wiring substrate according toclaim 4, wherein the insulating layer is formed such that a content rateof the inorganic filler particles in the insulating layer is in a rangeof 65% to 80%.
 20. The wiring substrate according to claim 4, whereinthe insulating layer has recesses on the surface of the insulating layerand is formed such that the first inorganic filler particles ispartially in the recesses on the surface of the insulating layer andthat the part of the metal film is in deepest parts of the recesses.